(a) Field of the Invention
The present invention relates to a frequency hopping method in a cellular mobile communication system using OFDM (orthogonal frequency division multiplexing). More specifically, the present invention relates to a frequency hopping method for providing the top performance of the OFDMA (orthogonal frequency division multiplexing access) system in the viewpoint of interference averaging, and a frequency hopping method in an OFDM system for providing a corresponding frequency hopping pattern designing method.
(b) Description of the Related Art
The OFDM method is a multi-carrier transmission technique for dividing all available bands into a plurality of narrow bands, modulating narrow-band subcarriers in parallel, and transmitting modulated subcarriers, and a small amount of low-rate data are allocated to the respective subcarriers.
Since the OFDM method uses orthogonal subcarriers, efficiency of frequency usage increases, and multi-path channels can be easily overcome by using a simple frequency domain equalizer with a single tap.
Also, since the OFDM method can be realized at high rates by using the FFT (fast Fourier transform), it is widely used as a transmission method for high-speed digital communication systems.
In particular, the OFDM method is applied to the mobile/wireless communication fields including WLAN, WMAN, and cellular mobile communication systems.
Cellular mobile communication systems based on OFDM are classified as OFDM-FDMA (OFDMA), OFDM-TDMA, and OFDM-CDMA according to multiple access methods of allocating wireless resources to a plurality of users.
Among them, the OFDMA method allocates part of the total subcarriers to each user to cover the plurality of users. To increase the frequency diversity gain and the frequency reuse rates, the OFDMA adopts frequency hopping for varying the allocated subcarrier groups with respect to time.
The frequency hopping in the OFDMA method is used together with channel encoding and interleaving to obtain a frequency diversity effect, as well as an interference averaging effect, the interference being provided from adjacent cells in the cellular environments.
FIGS. 1(a) and 1(b) show diagrams for describing the frequency hopping of subcarriers in the conventional OFDMA method.
Referring to FIG. 1(a), the vertical axis of the lattice is a frequency axis, and the horizontal axis is a time axis 10 representing a symbol period.
The reference numeral 11 on the frequency axis indicates a single subcarrier, the reference numeral 12 on the frequency axis shows a set of continuous subcarriers (i.e., a cluster) on the frequency axis, and the size is represented by multiplying the number of subcarriers in the lattice by a subcarrier frequency interval. The reference numeral 13 on the time axis is a unit for channel coding.
The frequency hopping OFDMA method on the cluster basis configures clusters, and randomly allocates the clusters for each symbol period on the basis of the configured clusters (that is, performs frequency hopping), thereby configuring a channel.
FIG. 1(a) shows that four adjacent subcarriers form a single cluster, showing an exemplified case of a four channel configuration format in the cell A, and FIG. 1(b) shows an exemplified case of a one channel configuration format in the cell B. It is assumed in FIGS. 1(a) and 1(b) that the cells A and B are adjacent or very near to each other.
As shown, the channel configurations of adjacent or near cells (the frequency hopping pattern) are to be different from each other so as to average the interference provided by the adjacent cells.
If two neighboring cells use the same hopping pattern, continuous and severe interference occurs between the identical channels. As to the channel a of the cell A and the channel e of the cell B, interference is generated for four symbol periods during 16 symbol periods that is a single channel coding period. That is, interference is not intensively generated to a single predetermined channel, but it is relatively generated to other channels (referred to as an interference averaging effect.)
As described above, the cells in a mobile communication network, based on the frequency hopping OFDMA, have their own specific hopping patterns, and the neighboring cells having different hopping patterns average the interference influencing the adjacent cells.
One of the conventional methods uses a pattern of pseudo random sequences for the above-noted frequency hopping pattern (a channel configuration format).
Degrees of interference that the channel e of the cell B imparts on the respective channels of the cell A will now be described when the frequency hopping pattern generated by the pseudo random sequences is created as shown in FIG. 1.
Interference occurs in the channels a and c of the cell A for four symbol periods, and the interference occurs in the channel b of the cell A for two symbol periods, thereby generating less interference. However, the interference occurs in the channel d for six symbol periods, thereby undergoing relatively very hard interference compared to the channels a, b, and c.
Frequent frequency collision between the specific channels arouses severe-interference to lower the system performance because of high BERs (bit error rates).
As shown, when sixteen subcarriers, four channels (a number of concurrent users), and a channel coding period of sixteen symbol intervals are provided, the case of inducing the interference to all the channels for four symbol periods is the best hopping pattern regarding the interference averaging.
Accordingly, the frequency hopping pattern made by the pseudo random sequences problematically fails to execute perfect interference averaging because the degrees of the interference influencing each other between the channels in the two adjacent cells are not uniform.
As to another conventional frequency hopping pattern designing method, a frequency hopping pattern designing method on the basis of the mutually orthogonal Latin square for overcoming the frequency hopping method on the basis of the above-noted pseudo random sequences will now be described.
The frequency hopping pattern designing method on the basis of the mutually orthogonal Latin square induces frequency collisions between all the channel pairs within two cells using different frequency hopping patterns the same number of times, thereby allowing obtaining of the complete interference averaging.
When the number of channels (the number of concurrent accessed users) is set as N in the frequency hopping pattern designing method on the basis of the mutually orthogonal Latin square, (N−1) different frequency hopping patterns for providing the complete interference averaging exist. When a mobile communication network includes a plurality of cells, it is necessarily required to reuse a frequency hopping pattern so as to allocate a single hopping pattern to each cell.
If the number N1 of the frequency hopping patterns is a big number, cells that are geographically distant to thereby generate much path loss and less interference with each other can be arranged so that the cells may use the identical hopping pattern.
If the number N1 of the frequency hopping patterns is a small number, arranging the same frequency hopping pattern between adjacent cells is unavoidable, which may give rise to severe interference between the users who use the same frequency hopping pattern, thereby severely lowering performance.
Therefore, while considering it very important to have a number of different frequency hopping patterns of more than a predetermined level, the above-noted method based on the mutually orthogonal Latin squares causes severe performance lowering when the number N of channels is small.
Further, the above-mentioned method provides a method for designing (N−1) mutually orthogonal Latin squares only when the number N of channels is a prime number or the square of a prime number. That is, the above-noted method on the basis of the mutually orthogonal Latin square cannot be applied to the case in which the number N has two or more prime numbers as divisors, such as 6, 10, 12, and 14.
Theoretically, when the number N of channels is six, no pairs of mutually orthogonal Latin squares exist. That is, if N=6, even two different orthogonal hopping patterns for enabling complete interference averaging cannot be made when following the above-noted method on the basis of the mutually orthogonal Latin square.